There are many injectors on the market where the aim is to have high degrees of functionality and automatic features, such as in connection with mixing, penetration, injection, setting of dose, priming and covering of the needle after use. At the same time there is a demand on robustness, repeatability and reliability regarding the function, which sometimes might be difficult to meet when dealing with complex multi functions involving many interacting components. When there further are demands on low production costs, especially for devices that are to be used only once, the picture becomes even more complex.
There are in the patent literature numerous solutions to injection devices, the bulk of which never enter the market due to that they do not meet the demands in one way or the other. There is therefore a continuous search for solutions that provide the desired functions that at the same time fulfil the functional and/or economical demands.
Many devices having multi-functions that work in sequence, such as for example mixing, followed by priming, followed by penetration, followed by injection, followed by withdrawal, have a subsequent sequence triggered at the end of a previous sequence, for example when the needle has reached full penetration depth, the injection sequence is triggered. Many of these functions and sequences are initiated and performed by tensioned power springs that, when released, moves components of the delivery devices with a speed that is often determined by the force of the spring deducted by eventual counter forces such as friction, hydraulic resistance etc.
However, there is in many instances a desire that a sequence is performed in a controlled manner and possibly at rather slow speeds. One example is the mixing of medicament in a dual chamber cartridge. For some types of medicament, if the liquid is forced into the chamber containing powder medicament too fast, a foaming process is started, which foam later is difficult to remove, and thus negatively affects the preceding injection. Another example is injection of certain types of medicament which tend to cause pain when injected too fast due to that the tissue cannot absorb the medicament as fast as the medicament is injected. Yet an example is that there should be a delay from the end of injection until the medicament delivery device is removed from the injection site.
A few attempts have been made regarding controlling sequences of medicament delivery devices. One example is disclosed in WO 88/10129, where a slow delivery injection device is described. It comprises an injector having a spring acting on a plunger rod via a piston arranged in a stationary housing. Inside the spring a damper is arranged, having a valve for setting the damping resistance of the damper and thus the injection speed. The speed control according to WO 88/10129 is rather bulky and is not intended for, nor is it practical to implement in, a hand-held medicament delivery device such as for example a pen injector. Another example is disclosed in WO2007020090 A1.
There is thus a need for speed control mechanisms that are reliable, flexible, contain few components and are compact in order to be able to be utilized in hand-held medicament delivery devices.